A large number of steel plate members are used in a car. Car weight is reduced in various manners to improve fuel consumption. The steel plate members are also targets of weight saving. That is, weight saving is required by reducing the thickness of the steel plate members and increasing strength.
The steel plate members used in a car are used for members for protecting passengers at the time of impact such as door impact beams or center pillar reinforcement.
Accordingly, such a steel plate member needs to surely maintain a predetermined strength.
In particular, when a steel plate member having a high strength used in a car is manufactured by using a hot stamping technology, the steel plate member is heated to a transformation point or higher, subjected to press forming by using a mold in the austenite area, and heat is extracted by a mold for martensite transformation in a general hot stamping technology.
It has been known that a steel plate member formed into a predetermined shape by using the hot stamping technology has a low toughness value since it remains to have a hardened structure.
Therefore, annealing process may be performed on the steel plate member or steel material after the processing by means of the hot stamping technology to improve the toughness value.
Furthermore, there have been proposed a high tension cold-rolled steel plate having a martensite single-phase structure and a tensile strength of 880 to 1170 MPa by appropriately setting the structure and heat treatment conditions of the steel material (for example, see Patent Document 1) and a high-strength steel having an average grain diameter of 10 μm or less in the martensite phase whose space factor is 80% or higher and having a tensile strength of 780 MPa or higher (for example, see Patent Document 2).                Patent Document 1: Japanese Patent No. 3729108        Patent Document 2: Japanese Patent Application Publication No. 2008-038247        